In a motor controller that controls driving of an electric motor of an electric power steering device mounted in a vehicle, an electric motor of a dynamo-electric brake device, and an electric motor for traveling of an electric vehicle or a hybrid vehicle, even if an abnormality occurs in a motor control system, continuous driving of the electric motor is demanded.
To meet the above demand, the motor windings of the multi-phase electric motor are configured in a duplex system, for example, electric currents are supplied from individual inverter units to the motor windings in the duplex system. If an off failure in which a switching unit in one of the inverter units is not conductive, that is an open failure occurs, a failure control unit identifies such a failed switching unit, controls other switching units except for the failed switching unit, and controls normal inverter units except for the failed inverter unit including the failed switching unit. A controller of a multi-phase rotary machine including the above-described failure control unit and an electric power steering device using the controller is proposed (for example, see Patent Literature 1).